1. Field of the Invention
The present invention relates to a polyvinyl alcohol film, and to a polarizing film and a polarizer that comprise the polyvinyl alcohol film.
2. Description of the Prior Art
A polarizer having the function of transmitting and blocking out light is a basic constitutive element in liquid crystal displays (LCD), along with the liquid crystal that functions as a switch for light therein. Regarding their application field, LCD were almost for small-sized instruments such as electronic calculators and wristwatches in the early days at the beginning of their development, but in these days, they have become much used in other various instruments such as notebook-sized personal computers, word processors, liquid-crystal color projectors, navigation systems for automobiles, and liquid-crystal TV. Given that situation, polarizer for these is required to be improved more so as to be free from a problem of irregular coloration than conventional ones.
In general, polarizer is fabricated by preparing a polarizing film substrate of a stretched and oriented polyvinyl alcohol film (this will be hereinafter referred to as “PVA film”, and its raw material, polyvinyl alcohol is a polyvinyl alcohol polymer and this will be hereinafter referred to as “PVA”) or of a polyene film (this is prepared by dehydrochlorinating a polyvinyl chloride film or by dehydrating a PVA film to thereby make it have a polyene structure therein), followed by applying thereto a polarizing element of iodine or dichroic dye.
Of such polarizing films that are obtained by stretching and orienting a PVA film, those having a polarizing element of iodine therein have good initial polarizing properties, but they are poorly resistant to heat and water and are therefore problematic in that their polarizing properties are worsened when they are used in a high-temperature and high-humidity environment. For improving the durability of such iodine-containing polarizing films, some methods have heretofore been taken into consideration. For example, PVA for these is modified or crosslinked, or PVA films are subjected to heat treatment, or the polarizing films themselves are protected with a polymer film of low moisture permeability. However, any of these methods could not still produce satisfactory results.
On the other hand, polarizing films having a polarizing element of dichroic dye therein are more resistant to heat and water than those with iodine therein, but are problematic in that their initial polarizing properties are not so good. This is because the molecular weight of dichroic dye is larger than that of iodine and, as a result, PVA films could not well adsorb such dichroic dye and are often unevenly dyed with it. A method was tried of introducing a cationic group into PVA, a raw material for PVA films, in expectation of improving the dichroic dye adsorbability of the resulting polarizing films (Japanese Patent Laid-Open No. 230606/1985). However, the polarizing films of the type that contain dichroic dye in the surface or inside the PVA film of such a cationic group-containing modified PVA are still problematic in that their water resistance is poor.
For fabricating polarizing films of good water resistance, a technique was proposed of using a film of an ethylene-vinyl alcohol copolymer having an ethylene content of from 1 to 24 mol % for the substrate of polarizing films (Japanese Patent Laid-Open Nos. 100115/1993 and 188624/1996). However, the polarizing films of the type of which the substrate is of a film of a polyvinyl alcohol polymer with an ethylene group introduced thereinto are also still problematic in that their polarizing properties are not good when they are dyed with dichroic dye, as so demonstrated in Comparative Example 6 given hereinunder.
The present invention is to provide a polyvinyl alcohol film that is useful for a raw material for polarizing films of which the advantages are that not only they have good water resistance and good polarizing properties but also they are free from the problem of irregular coloration, and to provide a polarizing film and a polarizer that comprise the polyvinyl alcohol film.